1. Field of the Invention
This invention relates to improved processes for the production of perchloroethylene by catalytic oxychlorination.
2. General Background and Summary of the Prior Art
Perchloroethylene is a highly useful chlorinated hydrocarbon which is used primarily for dry cleaning of fabrics and for solvent degreasing of metal parts. The usage of other chlorinated hydrocarbons such as trichloroethylene is expected to decline in view of antipollution regulations such as the Federal Clean Air Act of 1970, which categorizes certain chlorinated hydrocarbons such as trichloroethylene as photochemically reactive substances, contributing to air pollution, and hence subject to severe emission restrictions. On the other hand, perchloroethylene, a highly chlorinated hydrocarbon containing no hydrogen, is specifically exempted from such restrictions.
In the prior art, both fixed bed and fluidized bed catalyst systems have been employed for oxychlorination reactions to produce perchloroethylene. Typical of catalysts used in fixed bed systems are those comprising a metal having a variable valence, such as copper, supported on a carrier. Fixed catalyst beds have the drawback, however, of tending to develop hot spots with the consequent deterioration of the catalyst, and with attendant loss of active material, heat transfer problems and catalyst recovery requirements. Accordingly, a more recent trend has been toward the utilization of fluidized catalyst beds which permit better control of the reaction which is exothermic, corrosive and even explosive, while at the same time eliminating hot spot formation.
One type of oxychlorination catalyst which has been utilized commercially in fluidized form comprises a mixture of copper chloride and potassium chloride deposited on a carrier of "Florex", a highly calcined Fullers' earth, which is essentially a magnesium-aluminum silicate with minor proportions of iron, calcium, potassium and titanium oxides. The resulting catalyst contains between 6% and 12% copper by weight. A catalyst of this kind is described, for example, in U.S. Pat. Nos. 3,267,162 and 3,296,319.
One major drawback of the known oxychlorination processes for producing perchloroethylene is that 1,1,2-trichloroethane and/or unsym-tetrachloroethane are by-products. See, for example, U.S. Pat. Nos. 3,267,160, 3,393,132, 3,468,968, 3,709,950 and 3,926,847. These by-products make separation of the principal product, perchloroethylene, much more difficult as can be seen from the respective boiling points,
______________________________________ B.P. .degree.C., Atm. Press. ______________________________________ trichloroethylene 86.7 1,1,2-trichloroethane 113.9 perchloroethylene 120.8 unsym-tetrachloroethane 130.5 sym-tetrachloroethane 145.9 ______________________________________
Thus, there is only a 7.degree. C. difference in boiling point between perchloroethylene and 1,1,2-trichloroethane. When 1,1,2-trichloroethane is absent, the boiling point difference to the next lower boiling compound from which it would have to be separated is 34.degree. C. to the 86.7.degree. C. boiling point of trichloroethylene. Similarly, there is only a 10.degree. C. difference between perchloroethylene and unsym-tetrachloroethane. When unsym-tetrachloroethane is absent, there is a 25.degree. C. difference to the 145.degree. C. boiling point of sym-tetrachloroethane making its separation much less difficult.
Another drawback of most known oxychlorination processes is the presence of unacceptably high impurity levels in the perchloroethylene produced. For example, in the production of fluorocarbons, impurity levels must often be reduced to less than 50 parts per million. (See, for example, U.S. Pat. No. 3,751,494, which discloses a molecular sieve decontamination process to reduce impurity content). Such reductions of impurity levels are generally expensive and time-consuming processes.
A further drawback of most known oxychlorination processes is the strong tendency of hydrogen-containing impurities such as 1,1,1,2-tetrachloroethane to dehydrochlorinate at elevated temperatures. In the presence of metal surfaces, such as are generally found in chemical process equipment, these impurities or contaminants dehydrochlorinate releasing corrosive hydrochloric acid vapor (see, for example, U.S. Pat. No. 3,712,869). Similarly, 1,1,1,2-tetrachloroethane tends to undesirably decompose when subjected to operations such as distillation, rectification, evaporation or concentration.
The presence of acidic decomposition products has been recognized to be undesirable and deleterious in numerous patents dealing with dry cleaning and solvent degreasing applications employing perchloroethylene and the use of certain stabilizer systems primarily to counteract such acidic decomposition products (see, for example, U.S. Pat. No. 3,029,298).
Another drawback of most known oxychlorination processes is the toxicity level of saturated partially chlorinated hydrocarbon reaction products such as 1,1,2-trichloroethane and 1,1,1,2-tetrachloroethane as compared to perchloroethylene, (see "Threshold Limit Values, Chemical Substances in Workroom Air", published in National Safety News, October, 1974, pp. 95-104). Reduction in the levels of these impurities is desirable to minimize the potential of adverse effects resulting from repeated exposure of workers to such chemicals when the workers are involved in processes for producing or in processes which utilize the chemicals.
Yet another drawback of most known oxychlorination catalysts of the prior art is that it has not been possible simultaneously to obtain both a high yield of perchloroethylene in relation to carbon content of the feed, and good utilization of chlorine, whether furnished as HCl or chlorine or both. These known catalysts bring about the formation of substantial quantities of trichloroethylene, usually such that the mole ratio of perchloroethylene to trichloroethylene, under conditions that would give good chlorine utilization, is less than 3:1. Higher perchloroethylene to trichloroethylene mole ratios are typically obtained only at chlorine utilization as low as about 70%.
Experience has also shown that oxychlorination catalysts of the type described exhibit the undesired phenomenon of slugging. This abnormality in fluidization is a condition which has been described as one in which bubbles of gas coalesce to a size approaching the order of magnitude of the confining vessel. The particle layers, or slugs of granular solids, between such large gas bubbles, move upward in a piston-like manner, reaching a certain height, and then disintegrate, with the result that the catalyst rains down as individual particles or smaller aggregates. Slugging is undesirable from a purely mechanical standpoint in that stresses are produced in the reactor arising from shaking of the vessel. Moreover, the size of the reactor is limited owing to the unpredictability of the slugging phenomenon, which sometimes requires that the reactor be placed within a coolant bath consisting of a larger vessel, as described, for example, in British Pat. No. 1,123,477.
In most known oxychlorination processes, the reduction of the level of undesired by-products such as 1,1,2-trichloroethane or unsym-tetrachloroethane is accomplished by complex and costly post-oxychlorination procedures such as filtration, distillation, or recycle. See, for example, U.S. Pat. No. 3,751,494.
Other drawbacks of known oxychlorination processes have been recognized. For example, Fichtel et al, German patent specification No. 1,276,026, deals with oxychlorination processes requiring complex and costly distillation and recycle procedures to separate reactants which have not been converted in the oxychlorination reaction. Even with such procedures, however, such prior art processes result in the production of high levels of trichloroethylene (which as discussed herein, is undersirable) as well as the production of unacceptably high levels of undesirable chlorinated by-products.